Microcapsules are understood herein to be spherical beads with a diameter of from about 0.2 to about 5 mm, which contain a core surrounded by a membrane. The membrane protects the core and any active within the core from the surrounding medium. The use of microcapsules in liquid detergents is desirable not only for aesthetic reasons but also for the reduction of undesirable interactions between the core and liquid matrix, in particular decomposition of agents during long storage and incompatibility of individual components. Microcapsules also facilitate the handling of environmentally sensitive, biologically-active or hazardous materials such as enzymes, bleaching agents, redox materials, etc.
The use of microcapsules in liquid detergents is known from the literature. WO 00/46337 discloses a liquid detergent composition containing greater than 5% by weight of surfactant and an encapsulate containing greater than 10% by weight of active material and a cross-linked anionic gum. The anionic gum is cross-linked with a polyvalent cation, in preferred embodiments sodium alginate is cross-linked with calcium cations.
Calcium cross-linked encapsulates can be sensitive to chelants, such as those found in detergent compositions. Chelants can sequester calcium ions eroding the encapsulates and at the same time reducing the amount of chelant available for the cleaning process. Other mechanisms that can destroy alginate-based encapsulates is the substitution of calcium by other ions which have higher affinity for the alginate anions, such as sodium ions. The latter gives rise to sodium alginate which is soluble in water and therefore can lead to dissolution of the encapsulates.
Cross-linked encapsulates generally have a high porosity, not being suitable for low molecular active materials which can permeate through the pores.
If the level of cross-linking agent were such as to provide storage stability within the detergent composition, there is a risk that the encapsulates would not dissolve in the cleaning water, because calcium alginate is insoluble in water. In addition the calcium of the washing water may bind to the alginate giving rise to deposits on the cleaned articles.
WO 98/11870 discloses a liquid personal cleansing composition containing an encapsulated lipophilic skin-moisturizing agent. The lipophilic skin-moisturizing agent is encapsulated within a complex coascervate comprising a polycation and a polyanion. Encapsulates of this kind are not suitable for use in detergent compositions because the coascervate is so strong as not to release the active agent during the washing process.
WO 02/055649 discloses a method for the production of microcapsules containing washing and/or cleaning substances with semi-permeable capsule shells (membranes) by means of complexing suitable polyelectrolytes.
A problem found with microcapsules having semipermeable membranes is that part of the core material, including the active materials can leach out with time. This can give rise to interaction among incompatible ingredients and structural changes on the capsules.
Semi-permeable type microcapsules known from the literature do not seem to be strong enough to withstand liquid detergent manufacture processes and transport and at the same time be capable of breaking in use to release the core without leaving residues.
A requirement of liquid detergents containing microcapsules is that the microcapsules should be stably suspended in the liquid matrix. This brings further complexity to the design of the liquid matrix, usually requiring the use of a structurant or a thickening agent that may interact with other ingredients in the liquid matrix potentially impairing the chemical stability of the detergent.